As is well known in the art, fluorescent lamps come in all sorts of different tones or colors of white. Even though they all appear to be white, their color temperature varies anywhere from 2500 to about 6000 or even 8000 and 10,000 Kelvin (herein defined as degrees Kelvin). Herein "color temperature" is related to the temperature of black body which would give an equivalent tone of white light. In general, the lower the color temperature the redder the tone of the white light, and conversely the higher the color temperature the bluer the tone of the white light. There is no specific component in the lamps having a temperature equal to the color temperature--the term is a standard used in the industry to compare the color of various fluorescent (and for that matter incandescent) lamps. The drive for different colors of fluorescent lamps derives from our familiarity with the redder, warmer incandescent lamps and our desire to have the more efficient fluorescent lamps mimic this warmer light in certain instances. This is due to the fact that the market requirements differ greatly as to the degree of whiteness that is required for different situations. For example, offices use mostly high color temperature fluorescent lamps somewhere in the vicinity of 4100 or even 5000 Kelvin. Part of the reason for the higher color temperature requirements is that these lights tend to be somewhat closer to sunlight and therefore they induce alertness and crisp daylight ambiance or atmosphere. On the other hand, in applications where somewhat softer moods or after work atmosphere is more appropriate the color temperature of the light source is typically reduced to about 2500, 2700, or 3000 Kelvin. Those lamps tend to give a light color which is somewhat closer to sunset or dusk or to incandescent lamps that people are used to at home.
Discharge lamps with different color temperatures are obtained by blending different phosphors which under identical ultraviolet excitation give somewhat different colors. Therefore, a discharge lamp must be replaced by a lamp with a different phosphor blend to produce a different color light. The color of that lamp is fixed and determined by the choice of the phosphors, and that is the reason different color temperature lamps are on the market in separate bulbs.
Generally speaking, 80% or so of the sales of discharge lamps is for lamps with color temperature range from about 3000 to 5000 Kelvin. This 2000 Kelvin range provides a quite perceptible range of different colors. However, there are some sales for lamps with a color temperatures below 3000 Kelvin, and some sales for lamps with color temperatures well above 5000 Kelvin.
Typically, residential applications tend to prefer the lower color temperature fluorescent lamps either in the circleline or in the compact fluorescent configuration. The compact fluorescent lamps (CFL) that penetrate the residential market have color temperature in the 2700 to 3100 Kelvin range which gives a reddish quality to the white light. The content of red in these residential lamps is higher than the lamps found in offices or other such business applications. In the residential market of today, the available varieties of colors is acceptable, in fact, it is preferable. It is an object of the present invention to provide a color variable CFL for use in the kitchen area, the hall area, or in the rooms where lights stay on for a long period of time. In such settings, the residential customer is provided with the desirable (and marketable) advantage of changing the color of the light without replacing the bulb to provide different moods during the course of the day and over different seasons.
Prior attempts to make a variable color temperature fluorescent lamp have, for one reason or another, never been commercialized. In many of these cases the structures of these ideas are not practical, economical, or not amenable to efficient manufacturing. The remaining cases have other performance limitations which preclude commercial success. For example, it is well known in the art of fluorescent lamps that if one increases the temperature of the lamp the amount of mercury, which is in the vapor phase, increases substantially producing more of the blue mercury lines which increases the color temperature, and so the light appears more bluish. This does change the color of the light; however the life of the lamp is markedly reduced, and the additional energy supplied (to raise the temperature of the mercury) reduces efficiency (defined herein as the ratio of the light intensity emanating from the lamp compared to the electrical power supplied to the lamp).
Another attempt to provide variable color light from discharge lamps has been to use multiple lamps of different color temperatures side by side and/or mixed in a fixture. In order to use such a fixture, one lamp of one color temperature is fully energized and the other is not fully energized. By changing the power distribution between the two lamps, e.g. a low temperature (reddish) and a high temperature (bluish) lamp, it is possible to make the fixture emit light of different colors. This is a brute force approach whereby the lamps are not deployed at their full efficiency. Both lamp life and the efficiency are reduced when lamps are operated in this mode. Furthermore, one would need to sell a whole fixture with a variety of lamps in order for this variable color to be deployed. Another disadvantage of this approach is that one end of the fixture emanates a different color than the other end of the fixture due to the physical position of the two lamps in the fixture. Also, since one lamp is not fully energized one end of the fixture is brighter than the other end in addition to the color difference. This approach, from an aesthetic point of view, is not an acceptable solution and it has not resulted in a successful product.
Another device that provides variable color light from discharge devices is shown in U.S. Pat. No. 5,363,019, entitled, VARIABLE COLOR DISCHARGE DEVICE, to Itatani et al., and assigned to Research Institute for Applied Sciences, of Kyoto Japan. This patent issued on Nov. 8, 1994. This inventive device used a mixture of two gases that, when excited, provide different color discharge light. The gases are controlled by electric fields.
It is an object of the present invention to overcome this limitation by providing a single variable color temperature lamp having a coating of a fixed blend phosphor or layers of such coatings on the lamp bulb. A related object is to provide a lamp with multiple coatings of different phosphors or combinations of phosphors or blends of such phosphors.
It is an object of the present invention to provide a lamp where the color can be changed without substantial loss of efficiency and/or life and to provide a practical system that can be manufactured with existing technology.
It is yet another object of the present invention to provide a variable color fluorescent lamp with a variable color temperature that extends from at least 3000 to 5000 Kelvin. A related object is to provide variable color temperature fluorescent lamps wherein each lamp may have a variable color temperatures range a few hundred to several thousand degrees Kelvin.